1. Field of the Invention
The present invention relates to an ink jet head that performs recording or the like on a recording medium by means of the small ink droplets that fly onto it. The invention also relates to a method for manufacturing such heads, and an ink jet apparatus provided with such head as well.
2. Related Background Art
The ink jet recording method is one of the so-called non-impact recording types. The features and advantages of this recording method are that the noise which is generated at the time of recording is small enough to be neglected, while recording is possible on various kinds of recording media at high speeds, and that fixation is also possible on an ordinary paper sheet without any particular treatment given to it, while highly precise images are obtainable at lower costs, among some other advantages. The ink discharge recording method has been rapidly and widely utilized in recent years not only for a printer serving as a peripheral device of a computer, but also, utilized for the printing system of a copying machine, facsimile equipment, word processor, or the like, with such features and advantages as described above.
As the ink discharge method for the general type of ink jet recording type is currently in use widely, there is the method that uses electrothermal converting elements (heaters), and also, there is the one that uses piezoelectric elements (piezo elements). It is possible for both of them to control the discharges of ink droplets by means of electric signals. The principle of the method that uses the electrothermal converting elements is such as to apply electric signals to each of the electrothermal converting elements in order to enable ink around each electrothermal converting element to be boiled instantaneously, and that each of the ink droplets is then discharged at high speeds by the utilization of phase changes of ink that generate the abrupt development of each bubble. Therefore, the method that uses the electrothermal converting elements makes it possible, as its remarkable advantage, to structure the ink jet head with the nozzles that can be formed integrally with ease.
Nevertheless, there is still rooms for improvement for this method, such as to eliminate the voluminal changes of flying droplets due to heat accumulation on the ink jet head, the influence of the cavitation exerted on the electrothermal converting elements at the time of defoaming, among some others.
To make such improvements, there have been proposed ink jet recording methods and ink jet heads as disclosed in the specifications of Japanese Patent Application Laid-Open Nos. 54-161935, 61-185455, 61-249768, 4-10940, and 4-10941, for example. The ink jet recording methods disclosed in these specifications are characterized in that bubbles created on the electrothermal converting elements in response to recording signals are arranged to be in the state where the bubbles are communicated with the air outside through the discharge ports of the head so as to enable ink between each of the discharge ports and electrothermal converting elements to be discharged almost completely. More specifically, a complete ink discharge of the kind becomes attainable by the provision of means for discharging ink droplets having a shorter distance between each of its electrothermal converting elements and discharge ports. With a recording method of the kind, it becomes possible to improve the voluminal stability of flying ink droplets, and the capability of discharging smaller droplets at higher speeds, as well as to improve the durability of electrothermal converting elements by eliminating the influence of cavitation. As a result, highly precise images can be obtained easily.
FIG. 6A is a view schematically showing one example of the fundamental mode of an ink jet head having the droplet discharge means which enables the bubbles created on the electrothermal converting elements in response to recording signals to be communicated with the air outside. This view is partly broken for the illustration on an appropriate surface. FIG. 6B is a cross-sectional view of the head, taken along line 6B--6B in FIG. 6A. This ink jet head comprises many numbers of electrothermal converting elements 1 arranged on an Si substrate 4; nozzle walls 6 that form the ink flow paths 12 each positioned corresponding to each of the electrothermal converting elements 1; and an orifice plate 5 having ink discharge ports 2 as an integrated member. Further, on the surface of the orifice plate 5, a water-repellent film 11 is formed. Also, on the Si substrate 4, an ink supply port 3 is open from its back side for supplying ink.
FIGS. 7A to 7I are cross-sectional views which schematically illustrate each step of manufacture of the ink jet head represented in FIGS. 6A and 6B. (These views correspond to the representation of FIG. 6B.) In other words, on the Si substrate 4 (FIG. 7A), which is provided with the electrothermal converting elements 1 and the driving wiring (not shown) on it, a soluble resin layer 7 is formed (FIG. 7B). Then, this layer is removed with the exception of the ink flow path pattern (FIG. 7C). Further, the resin layer 7 is covered by the covering resin layer (the resin material to structure the orifice plate 5 and the nozzle walls 6) (FIG. 7D). Then, the portions corresponding to the discharge ports are removed (FIG. 7E). Subsequently, the water-repellent agent is applied to the surface of the covering resin layer (that is, to the surface of the orifice plate 5) in order to form the water-repellent film 11 (FIG. 7F). With the masking provided for other portions than the discharge ports 2, the excessive water-repellent film 11 is removed in the interior of the discharge ports 2 (FIG. 7G). Also, the ink supply port 3 is formed on the Si substrate (FIG. 7H). Lastly, the resin layer 7 is eluted for the formation of each ink path 12 (FIG. 7I), and then, the electrical connection, and the like are arranged to enable the electrothermal converting elements to be driven. Thus, the ink jet head is obtained as shown in FIG. 6A.
For such a head as shown in FIG. 6A, the distance between each of the electrothermal converting elements 1 and discharge ports 2 is made shorter so that the bubble is communicated with the air outside. In other words, the thickness of the orifice plate 5 is made extremely small (8 .mu.m for the example shown in FIG. 6B). Also, in order to make the thickness small, the material that forms the orifice plate 5 and nozzle walls 6 should be the one which can be processed with ease comparatively. Usually resin material is adopted.
However, if the orifice plate 5 and nozzle walls 6 are formed by resin material (which is generally gas permeable), while the orifice plate 5 should be made thinner, the moisture in ink in the interior of the head is subjected to the easier evaporation to the atmosphere through the orifice plate 5. As a result, ink in the head may become overly viscous, and the print quality tends to be affected. Also, there is a fear that the air outside may enter the interior of the head to create bubbles. Such an influence of the kind may be exerted not only in the mode of the head where electrothermal converting elements are used, but also, in the mode where some other ink discharge principle is adopted, such as the use of piezo elements, when the orifice plate is formed by resin material.
Also, in the steps of manufacture shown in FIGS. 7F and 7G (after the application of water-repellent agent and the mask removal), the water-repellent agent may in some cases remain on the inner surface of the discharge ports 2. Then, the meniscus of ink is subjected to breakage by the presence of such residue of water-repellent agent, which may affect the print quality in some cases. Here, on the other hand, in accordance with the knowledge obtained by the inventors hereof, it is found desirable to apply the water-repellent film 11 up to the edges of the discharge ports 2 in order to obtain good print quality. However, in the step of manufacture shown in FIG. 7G, masking is provided for the portions other than the discharge ports 2 for the prevention of the water-repellent agent from remaining inside the discharge ports 2. This makes it difficult to allow the water-repellent film to be formed up to the edges of the discharge ports 2.